Elevated levels of plasma total homocysteine (tHcy), termed hyperhomocysteinemia (HHcy), are associated prospectively with increased incidence and mortality of many human diseases including coronary heart disease, stroke, dementia, Alzheimer's disease, diabetes, osteoporosis, cancer, neural tube defects and steatosis of the liver. The number and variety of diseases associated with elevated tHcy suggests that it may affect very basic cellular functions. Homocysteine is an intermediary metabolite produced from the hydrolysis of S-adenosylhomocysteine (SAH), which is a by-product of methylation reactions involving the methyl-donor S-adenosylmethionine (SAM). Studies from our lab and others show that elevations in tHcy in plasma are associated with elevations of intracellular SAH in tissues. Since SAH is a potent inhibitor of methyl-transfer reactions, such as those involved in the regulation of gene expression, this could point to a possible mechanism linking elevated tHcy to diverse disease states. Therefore, the overall goal of this proposal is to test the hypothesis that severe HHcy causes elevations in tissue SAH that results in inhibition of chromatin methylation and alters cellular gene expression in vivo. The proposed experiments will utilize an inducible genetic and a nutritional mouse model that each induce severe HHcy by a different mechanism. There are three specific aims: (1) Determine if HHcy is associated with elevated tissue SAH levels and global hypomethylation of DNA and histones. (2) Determine how HHcy affects gene-specific transcript levels, DNA, and histone methylation. (3) Determine if overexpression of SAH hydrolase can reverse the hypomethylation and gene expression phenotypes. Achieving these aims will lead to understanding the mechanism by which elevated tHcy causes disease and will give us new insight into potential interventions that could delay or eliminate the onset of homocysteine related diseases.